Umar will undergo surgery and will miss the forthcoming Test series against South Africa and the one-day series against New Zealand in November.
"He has suffered double injuries in his left knee which flared up during the first two Tests
The 22-year-old left-handed batsman scored 75 and 43 in Pakistan's second Test win over Bangladesh in Peshawar last week, but missed the third Test through injury.
"Doctors have said he will take 8 to 10 weeks to recover from post surgical rehabilitation," added Hasan.